1. Field of the Invention
This invention relates generally to CMOS output buffers, and more particularly to a power efficiency control circuit that tri-states the output buffer before every transition.
2. Description of the Prior Art
Any CMOS buffer with pull-up/pull-down transistors will consume both dynamic (Idyn) and short circuit (Isc) current. While the dynamic current is unavoidable and is required to drive a capacitive load, the short circuit current is wasted current and should be minimized or even eliminated for low-power operation. The short circuit current is current that is momentarily shorted to ground (instead of being used to charge or discharge the output load) during the middle of any input transition. The short circuit current occurs while both the upper output (UOP) driver and the lower output (LOP) driver are on simultaneously. The short circuit currents reduce the AC performance of the device by 1) increasing the fall and rise time of the output, due to the inability to utilize the full sourcing and sinking current capabilities of the UOP and LOP transistors; and 2) degrading the output signal integrity by inducing ground bounce on adjacent pins.
In view of the foregoing, it would be both beneficial and advantageous to provide a technique for eliminating short circuit power consumption associated with a CMOS output buffer in a manner that substantially increases the buffer operating efficiency. It would be further advantageous if the technique were implemented in a manner allowing for a reduction of power associated with the output buffer pre-driver stage.